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LLLin000 e0260a8188 feat: recover 'Figure N' heading prefix from PDF text layer for OCR-missed captions

When PaddleOCR fails to detect a standalone 'Figure N' / 'FIGURE N'
heading (rendered in a bold/small-caps font that the OCR engine doesn't
read), the caption body is captured as figure_caption_candidate but
lacks the figure number prefix. The new _recover_figure_heading_prefix()
function checks the PDF text layer (via existing page_pdf_lines_by_page
infrastructure — no extra PDF open) and prepends the heading.

Key logic:
- Scans PDF text lines on the same page for 'Figure N' headings
- Confirms by checking if the NEXT PDF line (by y-order) shares ≥15
  chars of common prefix with the OCR caption candidate text
- Only runs when _extract_figure_number() returns None (no figure
  number in existing caption text)
- Integrated into build_figure_inventory() before _is_formal_legend()
  check, so recovered captions enter the legend matching pool

Fixes 'FIGURE 1' loss in 5S7UI34M and 'FIGURE 5' loss in HQAQBSBP.
372 figure tests pass.

2026-07-03 01:46:40 +08:00

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28ALPCY7 — "Figure N" Lost-in-OCR Investigation

Paper

Title: 3D-Printed β-Tricalcium Phosphate Scaffold Combined with a Pulse Electromagnetic Field Promotes the Repair of Skull Defects in Rats
PDF: Liang et al., 2019, ACS Biomaterials Science & Engineering
Pages: 37 total

Methodology

For each page containing figures/tables, we compared:

PDF text layer (PyMuPDF page.get_text()) — search for "Figure N"/"Table N" headings
OCR role-index (block_trace.csv from the PaperForge pipeline) — check for figure_caption/table_caption role

Pages with Figures/Tables (source: figure_table_ownership_summary.json)

Page	Content	PDF has heading?	OCR has caption block?	Status
9	Table 1 (primer sequences)	`Table 1.`	b5 table_caption	PRESERVED
12	Figure 1 (SEM images)	`Figure 1.`	b8 figure_caption	PRESERVED
13	Figure 2 (LIVE/DEAD staining)	`Figure 2.`	b10 figure_caption	PRESERVED
15	Figure 3 (CCK-8 analysis)	`Figure 3.`	b4 figure_caption	PRESERVED
16	Figure 4 (ALP activity)	`Figure 4.`	b4 figure_caption	PRESERVED
17	Figure 5 (ALP staining)	`Figure 5.`	b11 figure_caption	PRESERVED
19	Figure 6 charts (ALP/Runx2/OPN)	NO heading (charts only)	b4/b6/b8 fragment captions	Heading on page 20
20	Figure 6 caption (qRT-PCR)	`Figure 6.`	b3 figure_caption	PRESERVED
22	Figure 7 (3D reconstruction)	`Figure 7.`	b4 figure_caption	PRESERVED
23	Figure 8 (BMD analysis)	`Figure 8.`	b5 figure_caption	PRESERVED
27	Figure 9 (HE staining)	`Figure 9.`	b5 figure_caption	PRESERVED
36	TOC graphic	`Table of Contents graphic`	b3 table_caption_candidate	PARTIAL

Key Finding: No "Figure N" headings are lost from PDF text layer

All 10 numbered figure/table headings present in the PDF text layer are correctly captured in the OCR role-index. There is zero cases where a "Figure N"/"Table N" string exists in the PDF text but is absent from OCR blocks.

1. Figure 6 heading is on a different page than the figure assets

The figure caption "Figure 6. qRT-PCR analysis..." is on Page 20
But the actual figure chart images (ALP, Runx2, OPN expression) are on Page 19
The figure_table_ownership incorrectly maps Figure 6 to "page": 19 (the legend_block_id=3 on page 19 is actually a body paragraph, not the caption)
The three chart blocks on page 19 (b5=media_asset/chart, b7=media_asset/chart, b9=figure_asset/chart) have only fragmentary sub-panel labels ("ALP", "Runx2", "OPN") instead of the full Figure 6 heading

2. Pages 25-26: Full-page images with no text layer -> media_asset with no caption matching

Pages 25 and 26 contain full-page images with NO text layer (only page numbers and journal header in PDF text)
The OCR pipeline correctly identified these as media_asset blocks (6 total across pages 25, 26, 27, 36):
- Page 25: b3, b4, b5, b6 = media_asset/image (4 blocks)
- Page 26: b3, b4 = media_asset/image (2 blocks)
- Page 27: b3 = media_asset/image (1 block)
These are NOT caption-matched — they remain as unmatched/uncaptioned media_asset blocks
The vision audit confirms these are real figures/charts that "should have caption"
Cause: Captions are embedded within the image pixels, not as separate PDF text — the OCR pipeline cannot extract text from images

3. Page 17: Mixed media_asset + figure_asset blocks

Page 17 has both media_asset (b3-b6, 4 image blocks) and figure_asset (b9-b10, 2 image blocks)
The media_asset blocks have no figure_number assignment — they are uncaptioned
The figure_asset blocks are matched with figure_caption block b11 (Figure 5)
This suggests sub-panel images within Figure 5 are classified as media_asset, not figure_asset

4. Total uncaptioned figure assets

4 unmatched figure assets across the paper
8 unmatched table assets across the paper
2 unresolved figure clusters (pages 17 and 19) where near-neighbor caption matching failed

Summary

Metric	Count
Numbered figure/table headings in PDF text	10
Figure/table headings correctly captured in OCR	10
Figure N headings LOST in OCR	0
Media_asset blocks (images without caption matching)	18
Pages with media_asset but NO caption on same page	25, 26
Unmatched figure assets	4
Unmatched table assets	8

Conclusion

The "Figure N" lost-in-OCR problem does not affect paper 28ALPCY7 in the traditional sense. All extractable figure/table headings from the PDF text layer are faithfully captured in the OCR role-index blocks. The images on pages 25-26 have their captions embedded within the image pixels (no separate text layer), so there is no "Figure N" text to lose — the OCR pipeline correctly classifies them as media_asset rather than figure_asset, but this means they remain uncaptioned in the structured output.

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